The present invention relates to a bulk acoustic resonator used as an RF component and to a filter element using the same.
As recent mobile phone terminals have been produced in multi-band configurations, duplexers and inter-stage filters have been required to have lower-loss and steeper attenuation characteristics than were previously required. A FBAR (Film Bulk Acoustic Resonator) is a bulk acoustic resonator utilizing the resonance of an elastic wave along the thickness of a piezoelectric film. A FBAR filter which is comprised of the FBARs connected in a ladder configuration has received attention as a filter capable of achieving low-loss and steep attenuation characteristics.
FIG. 19 is a cross-sectional view of a bulk acoustic resonator used in a conventional FBAR filter (see, e.g., Japanese Laid-Open Patent Publication No. 2002-251190). As shown in FIG. 19, the bulk acoustic resonator comprises: an acoustic reflector portion 104 formed on a substrate 101 made of silicon; and an acoustic resonator portion 108 formed on the acoustic reflector portion 104. The acoustic resonator portion 108 comprises: an upper electrode 107; a piezoelectric film 106; and a lower electrode 105 which are successively stacked in layers.
The acoustic reflector portion 104 typically comprises: low acoustic impedance layers 103 each having a thickness corresponding to a quarter of the resonance wavelength; and high impedance layers 102 each having a thickness corresponding to a quarter of the resonance wavelength and a higher acoustic impedance than a low acoustic impedance material, which are alternately stacked. The acoustic reflection characteristics are determined by the ratio of the acoustic impedance value (hereinafter referred to as the “acoustic impedance ratio”) of each of the high acoustic impedance layers to that of each of the low acoustic impedance layers. The acoustic reflection characteristics are improved by increasing the number of the low acoustic impedance layers 103 and the high acoustic impedance layers 102 which are alternately stacked. Thus, a bulk acoustic resonator with a reduced propagation loss can be achieved by pairing up materials having a high acoustic impedance ratio therebetween or by stacking a larger number of pairs.
In view of this, it has been general practice to use silicon dioxide having as a relatively low acoustic impedance value for the low acoustic impedance layers and use a metal material having an extremely high acoustic impedance value, such as tungsten or molybdenum, for the high acoustic impedance layers.
However, the bulk acoustic resonator comprising the conventional acoustic reflector portion which uses silicon dioxide for the low acoustic impedance layers and uses a metal material for the high acoustic impedance layers has the following problems.
First, a material that can be used for the high acoustic impedance layers is substantially limited to tungsten, molybdenum, or the like because silicon dioxide, which is the material of the low acoustic impedance layers, does have an acoustic impedance of about 1.3×107 kg/s·m2, though it is considerably low. In addition, when tungsten is used to form the high acoustic impedance layers, it is necessary to stack at least four low acoustic impedance layers and at least four high acoustic impedance layers.
The acoustic reflectivity of the acoustic reflector portion lowers as the acoustic impedance ratio is lower. Accordingly, when materials having a low acoustic impedance ratio therebetween are used for the low acoustic impedance layers and for the high acoustic impedance layers, it is necessary to increase the number of the low acoustic impedance layers and the high acoustic impedance layers which are alternately stacked.
However, when the number of the low acoustic impedance layers and the high acoustic impedance layers which are alternately stacked is increased, the number of process steps and fabrication cost are also increased undesirably. In addition, when the number of the stacked layers is increased, the upper surface of the acoustic reflector portion is rough, which leads to the problem of poor crystallinity of the piezoelectric film of the acoustic resonator portion formed on the acoustic reflector portion.
In the case where the acoustic reflector portion is formed by using insulating silicon dioxide for the low acoustic impedance layers and a conductive metal material for the high acoustic impedance layers, an electrical leakage path may be formed disadvantageously by capacitance components and resistance components produced in the acoustic reflector portion. When the electrical leakage path is formed, the problem is encountered that signal leakage occurs between the individual acoustic resonator portions formed in adjacent relation on the acoustic reflector portion and causes the occurrence of a signal loss.